1. Field of the Invention
The present invention relates to slurry useful for wire-saw slicing a silicon ingot, a compound semiconductor block, a quartz block or the like, and also-relates to a method of evaluating such slurry.
2. Brief Description of the Prior Art
After top and tail parts are cut off a silicon ingot produced by a pulling method or the like, the silicon ingot is processed in the steps of outer diameter grinding, orientation flat processing, etc., and then sliced to wafers of a predetermined thickness. Although a slicer equipped with an inner blade has been used so far for slicing the ingot, a wire-sawing machine using a piano wire is used for slicing a big-size ingot.
As shown in FIG. 1, a conventional wire-sawing machine has generally three grooved rollers 1-3, one of which is coupled to a driving motor 4. A wire 5 is pulled out of a wire reel 6, wound several times around the grooved rollers 1-3, and led to another wire reel 7. A tension is applied to the wire 5 by a tensioner 8, so that the wire 5 travels in a stretched state around the grooved rollers 1-3.
An ingot 9 to be sliced is mounted to a holder 10 using a mounting jig located between the grooved rollers 1 and 2. The ingot 9 is sliced to a plurality of wafers by the wire 5. During slicing, slurry 11 is supplied to the wire 5 so as to facilitate slicing motion. The slurry 11 is fed from a slurry tank 12 through a supply pipe 13 and a nozzle 14 to the wire 5, recovered in a pan 15, and then returned to the slurry tank 12. The slurry 11 is circulated between the slurry tank 12 and a heat exchanger 16, in order to cool the slurry 11.
Such the slurry 11 is fed to inner parts of the ingot 9 in the state that grits are uniformly dispersed in a coolant, in order to slice the ingot 9 to a plurality of wafers with the wire 5. It is also an important factor that the grit-dispersing state is stable enough to avoid fluctuation of slicing conditions. In addition, the slurry 11 remaining on wafers cut off the ingot 9 shall be easily removed away from surfaces of the wafers.
Such slurry for wire-saw slicing is merely evaluated by viscosity, specific gravity and a pH value. The viscosity is commonly measured by a axial cylinder type viscometer 30 (Type-B viscometer) as shown in FIG. 2. An inner cylinder 31, which is hung from a stator 34 with a metal rod 33, is concentrically located in a vessel 32 (an outer cylinder). Sample slurry S is poured in a space between the inner cylinder 31 and the outer cylinder 32, and the outer cylinder 32 is rotated at a predetermined rotation speed. A force, which is corresponding to viscosity of the sample slurry S and along the rotating direction of the outer cylinder 32, is applied to the inner cylinder 31 by the rotating motion of the outer cylinder 32. Due to the force, a torsional moment (a torque) is applied to the metal rod 33. A scale disc 35 provided at the metal rod 33 is rotated in proportional to the torsion value. The viscosity of the sample slurry S is judged by reading a rotation angle of the scale disc.
Specific gravity is measured by a float balance-type hydrometer. A pH value is measured by an electrode-type pH meter.
Properties measured in these ways do not accurately represent an actual state of the slurry 11 which is being used for wire-saw slicing the ingot 9, so that slicing performance can not be forecast from those properties. Use of improper slurry sometimes causes various defects such as breakdown of the wire 5, damage of wafers, sticking of wafers cut off the ingot 9 and accumulation of the slurry 11 in the wire-sawing machine. In this regard, the slurry is evaluated by an actual ingot-slicing test. However, the test for researching ingot-slicing conditions needs plenty of time and cost. Such the troublesome test shall be avoided accounting the tendency that the ingot 9 is bigger and bigger in diameter.
The present invention is accomplished for elimination of the aforementioned problems and aims at a slurry suitable for wire-saw slicing without necessity of an ingot-slicing test by measuring flow characteristics (rheology) of slurry effective for ingot-slicing.
According to the present invention, slurry for slicing an ingot is evaluated by measuring viscosity of the slurry under conditions different in a shear speed using a cone and plate type viscometer. Flow characteristics (rheology) of the slurry is calculated from measured viscosity values.
The viscosity of the slurry measured by the cone and plate type viscometer is useful as a value for forecasting fluidization of the slurry during wire-saw slicing an ingot. Especially, such slurry, which has viscosity adjusted to approximately 400-700 mPaxc2x7second at a shear speed of approximately 2/second and to approximately 50-300 mPaxc2x7second at a shear speed of approximately 380/second, is suitable for wire-saw slicing an ingot.